Фазовая диаграмма системы Na-O

Na-O (Sodium-Oxygen)
H.A. Wriedt
The equilibrium solid phases of the Na-O system are (1) the terminal cph solid
solution, (aNa); (2) the terminal bcc solid solution, (bNa); (3) the fcc oxide
Na2O; (4) the hexagonal peroxide Na2O2-I; (5) the noncubic, but otherwise
structurally identified, peroxide Na2O2-II; (6) the orthorhombic superoxide
NaO2(III); (7) the cubic superoxide NaO2(III); (8) the fcc superoxide NaO2(I);
and (9) the bct ozonide NaO3. A third form of the peroxide, Na2O2-Q, with an
unidentified structure, might be an equilibrium phase [61Tal], but this
suggestion is uncomfirmed.
A complete phase diagram for the condensed Na-O system has not been published.
The assessed diagram is mainly schematic, because most phase boundaries and
temperatures or forms of the three-phase equilibria have not been determined.
For the condensed system at compositions up to 75 at.% O, there are twelve
possible three-phase equilibria, few of which have been observed, and nine
possible transitions.
At 0.1 MPa hydrostatic pressure, aNa is stable up to -237C [56Bar], and from -
237 C to the melting point of 97.83 C, bNa is stable. The solid solutions (
aNa) and (bNa) saturate with respect to Na2O at very small oxygen
concentrations, which have not been determined. The (bNa) solidus is also
undetermined.
Na2O exhibits only the fcc structure. On its Na-rich side, Na2O is in
equilibrium with (aNa) below -237 C, with (bNa) from -237 to 98 C, with L1
from 98 to 1130 C, and with L2 from 1130 C to its congruent melting point
at 1134 C. Except for the latter temperature, these values are approximate.
The 1130 C reaction has been identified as monotectic, but, except as noted,
the types of the other reactions among the three condensed phases have not
been established.
On its O-rich side, Na2O is in equilibrium with Na2O2-I below ~512C, with
Na2O2-II from ~512 C to the probable eutectic temperature of ~570 C, and
with L2 from ~570 to 1134 C. The range of Na2O composition is undetermined,
but is probably narrow, at least at low temperatures.
Although a third known polymorph of the peroxide, Na2O2-Q, may be stable,
there is insufficient information to include it in the assessed diagram, which
shows only Na2O2-I and Na2O2-II as stable peroxide polymorphs. At temperatures
below 512 C, where it undergoes a polymorphic transformation to Na2O2-II at 0.
1 MPa hydrostatic pressure, Na2O2-I is in equilibrium with Na2O on its Na-rich,
and on its O-rich side, it is in equilibrium with NaO2(III) below about -77
C, with NaO2(II) from about -77 C to about -50 C, with NaO2(I) from
about -50 C to the temperature (assumed to be below 512 C) of the required
but apparently unobserved Na2O2-I + L2 + NaO2(I) equilibrium, and with L2
above.
No observations have been reported of the three-phase equilibria at about -
77 or -50 C. On its Na-rich side from ~512 to 570 C, Na2O2-II is in
equilibrium with Na2O; otherwise, on both sides, it is in equilibrium with L2
below its 675 C melting point. Although Na2O2-II may exhibit an appreciable
variability of composition, systematic studies of the phase boundaries and of
its compositions as a function of temperature and oxygen fugacity have not
been reported. Decomposition of Na2O2-I is supressed to at least 500 C by
oxygen gas at 0.1 MPa pressure (fugacity) [47Bun].
Although the superoxide NaO2 exhibits only three crystal structures, a
magnetic transition from NaO2(III) to NaO2(IV) without change of crystal
symmetry occurs at low temperature. On their Na-rich sides, the NaO2 phases
are in equilibrium with Na2O2-I or with L2. On their O-rich sides, the NaO2
phases are in equilibrium only with NaO3, except for NaO2-I, which also may
coexist with L2. Appreciable variability in the composition of NaO2-I has been
detected, but systematic studies of the phase boundaries have not been
reported.
The highest known oxide, the ozonide NaO3, has been reported to occur in two
varieties exhibiting differences in chemical behavior, but structural
differences or an equilibrium transformation have not been observed. The
melting points and deviations from the stoichiometric composition are also
unknown. Na-rich NaO3 would be in equilibrium, according to temperature, with
each of the NaO2 modifications; on the O-rich side, the equilibria are unknown.
Only three liquidus segments have been investigated. Numerous measurements of
the L1 liquidus of Na2O have been made; the data of [73Nod] were used to
construct the curve below 500 C in the assessed diagram. The O-rich liquidus
of Na2O and the Na-rich liquidus of Na2O2-II were studied by [47Bun], but only
the data for the latter curve are possibly near the correct values.
47Bun: E.G. Bunzel and E.J. Kohlmeyer, Z. Anorg. Chem., 254(1-2), 1-30 (1947)
in German.
56Bar: C.S. Barrett, Acta Crystallogr., 9, 671-677 (1956).
57Fop: H. F”ppl, Z. Anorg. Chem., 291, 12-50 (1957) in German.
61Tal: R.L. Tallman and J.L. Margrave, J. Inorg. Nucl. Chem., 21, 40-44 (1961).
73Nod: J.D. Noden, J. Brit. Nucl. Energy Soc., 12(3), 329 (1973).
Published in Bull. Alloy Phase Diagrams, 8(3), Jun 1987. Complete evaluation
contains 1 figure, 2 tables, and 144 references.
Special Points of the Na-O System